The long-term objective of the proposed research is to understand the cellular mechanisms underlying the detection and signaling of light stimuli by vertebrate retinal photoreceptors. The specific aims are to study: 1) the nature of the transduction stages between light absorption by visual pigment and ultimate generation of an electrical signal, 2) the involvement of sodium, calcium and other ions in transduction, 3) the involvement of cyclic nucleotides and related biochemical substrates in transduction, 4) the characteristics of the light-sensitive conductance, which gives rise to the ultimate electrical light response to be transmitted to higher neurons, 5) any possible fundamental differences between rods and cones in their transduction mechanisms, and 6) spontaneous excitation of visual pigments in darkness. Knowledge of visual transduction is important for understanding various disease states affecting photoreceptors and for devising remedies for them. The methodology will involve mainly electrophysiological recording, consisting in extracellularly recording membrane current from single rods and cones using a suction pipet, together with intracellular recording. The extracellular current recording method has unusual stability and high measurement resolution, and is very suitable for studying visual transduction, especially if employed in conjunction with pharmacology.